28VDC to 3.3 V without meltdown??

[Steve Myers]

I am trying to build a simple DC-DC voltage regulator that can
drop input power ranging from 5 to 28VDC, 1.5 AMP Max down to 3.3VDC
to run a micro. The maximum current needed at 3.3 is less than 750mA.

I went to National's website and entered in the paramerters and tried
using the suggested switcher (LM2595-3.3), but the "Webtherm" shows
the design will generate about 125deg F at normal operation.

Is the a cooler way to do this, or should I just use it as is?

Steve

 

[John Popelish]

Steve Myers wrote:

I am trying to build a simple DC-DC voltage regulator that can
drop input power ranging from 5 to 28VDC, 1.5 AMP Max down to 3.3VDC
to run a micro. The maximum current needed at 3.3 is less than 750mA.

I went to National's website and entered in the paramerters and tried
using the suggested switcher (LM2595-3.3), but the "Webtherm" shows
the design will generate about 125deg F at normal operation.

Is the a cooler way to do this, or should I just use it as is?

Steve

You can use a bigger heat sink than they specify, but you can also
just go through the process, again, but specify a higher maximum
current (but the same minimum current) to get a design with more
safety factor.

--
John Popelish

 

[CBarn24050]

125F doesn't seem too high, if you have a fan anywhere you could direct some of
the airflow, even a gentle breeze makes a big difference.

 

[Jeffrey Turner]

Steve Myers wrote:

I am trying to build a simple DC-DC voltage regulator that can
drop input power ranging from 5 to 28VDC, 1.5 AMP Max down to 3.3VDC
to run a micro. The maximum current needed at 3.3 is less than 750mA.

I went to National's website and entered in the paramerters and tried
using the suggested switcher (LM2595-3.3), but the "Webtherm" shows
the design will generate about 125deg F at normal operation.

Is the a cooler way to do this, or should I just use it as is?

You could try this circuit, just use a 3.9V Zener instead of the 5.6V
Zener shown.

http://www.romanblack.com/smps.htm

--Jeff

--
A man, a plan, a cat, a canal - Panama!

Ho, ho, ho, hee, hee, hee
and a couple of ha, ha, has;
That's how we pass the day away,
in the merry old land of Oz.

 

[Nemo]

In article <h13g50p82pkv64c0nl2h2kpulpipagkg22@4ax.com>, Steve Myers
<smyers@ihavenomail.com> writes

I am trying to build a simple DC-DC voltage regulator that can
drop input power ranging from 5 to 28VDC, 1.5 AMP Max down to 3.3VDC
to run a micro. The maximum current needed at 3.3 is less than 750mA.

I went to National's website and entered in the paramerters and tried
using the suggested switcher (LM2595-3.3), but the "Webtherm" shows
the design will generate about 125deg F at normal operation.

Is the a cooler way to do this, or should I just use it as is?

The max current rating of the input power supply is irrelevant, except
that simple switchers generally need to draw peak currents of twice the
average output current for a few microseconds. But that's what their
input capacitors are for.

Most switchers at least 85% efficient these days in the kind of
situation you describe. So if it is supplying 3.3 x 0.75 watts, it is
dissipating 15% of that itself; which is about 0.4 watts. This is tiny.

Whilst the un-heatsunk package may not be able to handle this (most SO8
packages can dissipate, what, half a watt or so but it is best to run
cool if possible)... you should be able to get by with a tiny heatsink.
The cheapest heatsink is say one square inch of copper tracks. Make sure
the switcher IC is soldered or screwed down to this (I'm not sure what
package the LM2595 is) - basically you want decent thermal contact. Make
sure the heatsink area is tinned copper, not covered by solder resist
(which halves its heatsinking effect - insulates it from the air). Make
sure the copper area is reasonably exposed to air. And you'll be fine.
You'd probably be fine with half a square inch.
--
Paul Honigmann

 

[Chaos Master]

No B.O. número <20040317113658.20709.00001531@mb-m24.aol.com>, de 17 Mar 2004
16:36:58 GMT, consta que CBarn24050 escreveu no sci.electronics.basics :

125F doesn't seem too high, if you have a fan anywhere you could direct some of
the airflow, even a gentle breeze makes a big difference.

And if the OP used one of those BIG CPU coolers used by overclockers on their PC
processors? Yes, it would be somewhat noisy, but...


--
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irc.brasnet.org - #xlinuxnews and #poa
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